The technical field of this invention is that of nondestructive materials characterization, particularly quantitative, model-based characterization of surface, near-surface, and bulk material condition for flat and curved parts or components using magnetic field based or eddy-current sensors.
Characterization of bulk material condition includes (1) measurement of changes in material state, i.e., degradation/damage caused by fatigue damage, creep damage, thermal exposure, or plastic deformation; (2) assessment of residual stresses and applied loads; and (3) assessment of processing-related conditions, for example from aggressive grinding, shot peening, roll burnishing, thermal-spray coating, welding or heat treatment. It also includes measurements characterizing material, such as alloy type, and material states, such as porosity and temperature. Characterization of surface and near-surface conditions includes measurements of surface roughness, displacement or changes in relative position, coating thickness, temperature and coating condition. Each of these includes detection of electromagnetic property changes associated with either microstructural and/or compositional changes, or electronic structure (e.g., Fermi surface) or magnetic structure (e.g., domain orientation) changes, or with single or multiple cracks.
A specific application of these techniques is the inspection of high-strength steel components with the goal of measuring applied and residual stresses and detecting early stage fatigue damage and hydrogen embrittlement. Highly stressed aircraft components, such as landing gear components, require the use of steels such as 4340M and 300M heat treated to very high strength levels. The integrity of these components is critical to the safe operation of aircraft and for maintaining readiness of military aircraft. However, unintentional loading of these components, such as a hard landing, can impart residual stresses that compromise the integrity of the component. Similarly, the mechanical properties of these ultra-high strength steels can be seriously degraded as a result of the ingress of hydrogen. Hydrogen ingress can occur during pickling or plating operations and also during cleaning with citric acid based maintenance solutions. The resulting hydrogen embrittlement is unpredictable and can cause catastrophic failure of the component.
The detrimental effects of hydrogen on material properties and component integrity have been observed in a wide range of metals, as described for example in Interrante and in Hydrogen in Metals. Management of high-strength steel components embrittled by hydrogen is made more difficult by the fact that failures are typically delayed, occurring some time after ingress of atomic hydrogen. The delay between exposure to hydrogen and failure of a high strength steel component depends on a number of factors. Among these are the levels of hydrogen concentration, tensile stress, temperature, stress gradients, and certain impurities in the steel, as well as the type, concentration, and size of certain crystal lattice defects and inclusions. Moreover, susceptibility to hydrogen embrittlement can vary significantly between different heats of steels and between different pours from a given heat, as described by Lawrence. Hydrogen concentration on the order of a few parts per million is sufficient to cause hydrogen embrittlement and delayed fracture. Once atomic hydrogen enters the steel, excess hydrogen atoms diffuse to inclusions, preexisting defects, and zones of high dislocation density. Some hydrogen atoms, as a result of stress-assisted diffusion, can cluster and form “platelets” leading to initiation of microcracks. When such platelets form in front of a crack tip, they facilitate crack extension. Critical regions where hydrogen cracks are more likely to initiate are notches or other stress raisers where local hydrogen concentration is higher due to enhanced diffusion into the triaxially stressed region in front of a stress raiser. Cracks at these critical locations initiate close to but beneath the surface.
A recent review of existing magnetic/electromagnetic, diffraction, ultrasonic and other methods for assessment of residual stresses in steel components by Bray highlighted strengths and weaknesses of the available methods. This review also indicated that practical and cost-effective methods for assessment of residual stresses as well as for monitoring of applied stresses over wide areas in steel components are not yet available. Typically, discrete strain gages are mounted directly onto the material under test (MUT). However this requires intimate fixed contact between the strain gage and the MUT and individual connections to each of the strain gages, both of which limit the potential usefulness for monitoring stress over large areas. Possible correlations between magnetic properties and stresses in ferromagnetic materials have been studied for over 100 years, as reviewed by Bozorth. Magnetostriction effect data suggests that, depending on the magnitude and sign of the magnetostriction coefficient, correlation between stress and magnetic permeability within certain ranges of the magnetic field should be present. However, attempts to use conventional inductive, i.e., eddy-current sensors for assessment of residual stresses as well as for a number of other applications have shown serious limitations, particularly for complex geometry components.
Conventional eddy-current sensing involves the excitation of a conducting winding, the primary, with an electric current source of prescribed frequency. This produces a time-varying magnetic field at the same frequency, which in turn is detected with a sensing winding, the secondary. The spatial distribution of the magnetic field and the field measured by the secondary is influenced by the proximity and physical properties (electrical conductivity and magnetic permeability) of nearby materials. When the sensor is intentionally placed in close proximity to a test material, the physical properties of the material can be deduced from measurements of the impedance between the primary and secondary windings. Traditionally, scanning of eddy-current sensors across the material surface is then used to detect flaws, such as cracks.
In many structures, fasteners such as bolts and rivets are used to hold various structural elements together. These fasteners also help to transfer the mechanical load on the structure between the various elements. The number, type, and size of fasteners used in a given structure are generally designed so that the loads on the fasteners are not excessive. This is accomplished using model stress calculations for expected applied loads and the geometries and mechanical properties of the various elements. However, unanticipated loads on the structure or local changes in the structure due to corrosion and/or fatigue can lead to excessive cyclic and sustained stresses, and fatigue failures of the fasteners and structural elements, which can compromise the integrity of the structure.
As an example, consider the accumulation of damage at multiple sites on aging aircraft. The cyclic loading of these aircraft over extended periods can lead to the formation of cracks at multiple locations, such as between fasteners in a lap joint. Individually, the growth of one of these cracks to the next fastener may not compromise the structural integrity, but it can alter the load distribution among the fasteners. This load redistribution around the nearby fasteners can accelerate crack propagation, if the cracks are already present, or cause initiation and propagation of other cracks. In either case, the capability to monitor the stress distribution in the fasteners can provide vital information about the load on the fastener and the structure.